Actually my preference for ST115 is inboard and enjoy the reactive aspect of the ST ride, but logically outboard produces a more traditional locked down rail power/slalom stance, which also helps when using bigger fins in ST115.

Front/Middle/Back depends somewhat on rider height (stance) and water conditions, but Middle is a good starting point - allows a good mix of control and speed across the fun stuff (and obviously the average of many individual preference/s during development / testing).